This invention relates to a method and apparatus for charging a plurality of rechargeable batteries connected in series, and more particularly to a charging method suited to nonaqueous rechargeable batteries.
The rechargeable battery, which can be repeatedly charged and reused, suffers a severely degraded performance when it is over-charged. Therefore, in the case of one rechargeable battery, the lithium ion rechargeable battery, charging begins with constant current charging and switches to constant voltage charging when the voltage approaches a prescribed value. This shortens charging time and prevents over-charging. Since over-charging does not occur when battery voltage is low, the battery is quickly charged with a fixed current until the prescribed battery voltage is approached. Charging is then switched from constant current to constant voltage to prevent over-charging. If the battery were charged with constant voltage from the beginning, enormous charging currents would flow in the early stages of charging. On the other hand, if constant current charging were continued to the end, over-charging would easily become a problem.
This type of battery charging method that begins with constant current charging followed by constant voltage charging is described in Japanese Patent Disclosure 4-183232 (1992). This charging method is realized by the circuit shown in FIG. 1.
The battery charger shown in this figure charges the rechargeable battery B in the following manner. First, at the beginning of charging when the voltage of the rechargeable battery B is low, the output of operational amplifier (op-amp) 1 is positive (+). This is because the voltage at the non-inverting (+) input terminal of the op-amp is higher than that at the inverting (-) input terminal. Consequently, the base of transistor TR1 does not get connected to ground through the op-amp 1, and the series connected battery B is charged by the constant current charging circuit made up of transistors TR1 and TR2.
As the rechargeable battery B becomes fully charged and its voltage rises, the output voltage of the op-amp goes negative (-). Due to this, the base of transistor TR1 is connected to ground through the op-amp 1 turning transistor TR1 off.
When the rechargeable battery B voltage is below the prescribed reference voltage, the output voltage of the op-amp is (+) and transistor TR1 conducts to charge the battery. In other words, in this fashion the battery is charged without exceeding a fixed voltage (constant voltage charging).
In the case where a plurality of batteries connected in series are charged using the battery charger shown in FIG. 1, balanced charging of each battery can be accomplished if the capacity and internal resistance of all the batteries always remains the same. In actuality there is some variation in battery capacity and internal resistance. Further, even when internal resistance is initially the same, the internal characteristics and internal resistance change as the battery progresses through many charging cycles. As a result, the balance between each battery in a series connection is destroyed and over-charging occurs.
For example, a lithium ion rechargeable battery becomes fully charged at 4.2 V. When two lithium ion rechargeable batteries are charged in a series connection, properly operating batteries can be fully charged by setting the reference voltage for constant voltage charging at 8.4 V. However, if the battery capacity of the two batteries is not the same, one battery can be at 4.5 V while the other battery can be 3.9 V. In this case, the rechargeable battery with a voltage up to 4.5 V has been over-charged and its performance will be severely degraded.
Further, in the case where the internal resistance of the two batteries is not the same battery voltage during charging will rise more for the battery with larger internal resistance since the charging voltage (equal to the charging current times the internal resistance) is greater for that battery, and the high internal resistance battery will be over-charged. The internal resistance of this battery will further increase with each charging resulting in further over-charging and performance reduction.
Still further, nonaqueous rechargeable batteries such as the lithium ion rechargeable battery produce gas from a secondary reaction that dissociates the electrolyte when the prescribed voltage is exceeded and over-charging occurs. This increases the battery internal pressure as well as degrading performance. This internal pressure increase can cause the tightly sealed lithium ion rechargeable battery casing to burst. For these reasons the detrimental effects of nonaqueous rechargeable battery over-charging are extremely serious and prevention of over-charging 1s extremely important.
The present invention was developed considering the above mentioned points. It is thus the object of the present invention to provide a rechargeable battery charging method and apparatus that can charge each of a plurality of rechargeable batteries connected in series close to full charge without over-charging and with superior charge-discharge cycling characteristics.
The above and further objects and features of the invention will. more fully be apparent from the following detailed description with accompanying drawings.